1. Field of the Invention
The present invention relates to a sheet detecting device, and more particularly to a sheet detecting device reliably detecting a feeding state that sheets are doubly fed.
2. Description of the Background Art
A scanner, a printer, a copying machine, a printing machine, an ATM (automated teller machine), or the like, has a mechanism in which a bundle of sheets, such as papers or bank notes, are separated into each single sheet, and each separated one is fed sheet by sheet. In such a feeding, though each of the single sheets should be to be fed sheet by sheet, a doubles feeding occurs in the case where two or more sheets are erroneously fed while being partially or entirely superimposed one on another. In the case of the doubles feeding, it is necessary to give a user an alarm of the doubles feeding. In view of the doubles feeding, a doubles detector for detecting the doubles feeding is provided in each of those machines.
From the standpoint of the detection principle, the doubles detectors are categorized into a doubles detector of a level type as disclosed in Japanese patent No. 1725105 and a doubles detector of a phase type as disclosed in JP-A-52-40379.
In the doubles detector of the level type, a transmitter for transmitting an ultrasonic wave and a receiver for receiving the ultrasonic wave are provided in a feeding path through which the sheets are fed. The receiver receives the ultrasonic wave from the transmitter, through a sheet or members being fed (more exactly, the ultrasonic wave transmitted through the sheet or members), and the receiver outputs a signal corresponding to a reception level of the ultrasonic wave. When a level of the signal outputted from the receiver in the case of one sheet is compared with a level of the signal in the case of two or more number of sheets, the ultrasonic wave in the latter case is more attenuated than that in the former case, and hence the output level in the latter case is smaller than in the former case. For that reason, it is possible to judge whether or not the current sheet feeding is the doubles feeding by comparing the level of the output signal of the receiver is compared with a predetermined threshold level.
In the doubles detector of the phase type, the transmitter transmits a signal of a predetermined phase. The receiver receives a signal which is transmitted through a sheet or members. When a phase of the signal in the case of two or more sheet is compared with a phase of the signal in the case of one sheet, the phase of the signal varies more greatly in the former case than that in the latter case. For that reason, a doubles feeding may be detected by the utilization of the result of comparing a difference between the phases of the transmitting and receiving signals compared with a predetermined reference phase.
In the doubles detector of the level type, when the sheet is thin, its influence on the attenuation of the ultrasonic wave is small. Therefore, when the feeding of the one sheet is compared with the doubles feeding, a level difference between the receiving ultrasonic waves of the above two cases is not large. Therefore, the doubles detector has a disadvantage that it is very difficult to detect the doubles feeding in the case where the sheet is thin.
The doubles detector of the phase type has such a disadvantage that when the sheet is thick, the doubles detector of the phase type fails to detect the doubles feeding. More precisely, in a case where the sheet is thick, the attenuation of the ultrasonic wave is large. Therefore, in the case of the doubles feeding, the ultrasonic wave is greatly attenuated. As a result, a waveform of the ultrasonic wave is greatly deformed so as to fail to secure an S/N high enough to detect the periods of the ultrasonic wave, and the phase comparison becomes impossible.
Further, the phase type of the doubles detector has the following disadvantage. When surrounding temperature and atmospheric pressure vary, a propagation velocity of the ultrasonic wave also varies, thereby influencing a phase of the receiving signal. Therefore, the doubles detector cannot detect the doubles feeding correctly.
The present invention has been made in view of the above circumstances, and an object of the invention is to reliably detect the doubles feeding irrespective of the thickness of the sheet, and variations of surrounding temperature, atmospheric pressure and the like.
According to the present invention, there is a sheet detecting device comprising: transmitting means for transmitting a transmission signal; receiving means for receiving the transmission signal from the transmission means, through a feeding path through which a sheet is fed, and outputting a reception signal; generating means for generating a reference signal of a predetermined phase; phase comparing means for comparing a phase of the reception signal output from the receiving means with that of the reference signal generated by the generating means, and outputting a signal having a level corresponding to a phase difference therebetween; storing means for storing a predetermined reference level; and level comparing means for comparing an output of the phase comparing means with the reference level stored in the storing means, and outputting the result of the comparison.
In the thus constructed sheet detecting device, a transmission signal transmitted by the transmitting means is received by the receiving means, through a sheet. A phase of a reception signal output from the receiving means is compared with a phase of a reference signal, and a signal having a level corresponding to the phase difference is further compared with a predetermined reference level.
Before proceeding with the description of the present invention, description will first be given for clarifying the relationships of the means used in the description of the claim and the corresponding members or portions in the detailed description of the preferred embodiment. To this end, the technical features of the invention will be described in a manner that those means are attached with specific examples (one for each means) put in parentheses.
According to a first aspect of the present invention, there is provided a sheet detecting device comprising: transmitting means (e.g., ultrasonic-wave transmitter 1 in FIG. 1) for transmitting a transmission signal; receiving means (e.g., ultrasonic-wave receiver 2 in FIG. 1) for receiving the transmission signal from the transmission means, through a feeding path through which a sheet is fed, and outputting a reception signal; generating means (e.g., arithmetic operation unit 17 in FIG. 1) for generating a reference signal of a predetermined phase; phase comparing means (e.g., extracting unit 19 in FIG. 1) for comparing a phase of the reception signal output from the receiving means with that of the reference signal generated by the generating means, and outputting a signal having a level corresponding to a phase difference therebetween; storing means (e.g., threshold-value setter 20 in FIG. 1) for storing a predetermined reference level; and level comparing means (e.g., comparator 21 in FIG. 1) for comparing an output of the phase comparing means with the reference level stored in the storing means, and outputting the result of the comparison.
According to a second aspect of the present invention, in a sheet detecting device, the generating means includes a first holding means (e.g., phase holder 14 in FIG. 1) for causing a phase of the reception signal that is output from the receiving means when no sheet is present in the feeding path, to reflect on a phase of the reference signal.
According to a third aspect of the present invention, in a sheet detecting device, the generating means includes a second holding means (e.g., phase setter 15 in FIG. 1) for causing a phase of the reception signal, at a predetermined time, that is output from the receiving means when no sheet is present in the feeding path , to reflect on a phase of the reference signal, and a third holding means (e.g., phase setter 16 in FIG. 1) for causing a phase of the reception signal, at the predetermined time, that is output from the receiving means when one sheet is present in the feeding path, to reflect on a phase of the reference signal, wherein the generating means determines a phase of the reference signal in accordance with a difference between the sum of the phase stored in the first holding means and the phase stored in the third holding means, and the phase stored in the second holding means.